1. Field of the Invention
The present invention relates to a gradient index silicate glass, especially one readily produced by the sol-gel process.
2. Discussion of Related Art
The lens known as GRIN lens (gradient index lens) is a novel lens which has a refractive index distribution in its medium, so that not only refraction on lens surface but also refraction in the medium of the lens can be utilized. Especially, the radially gradient index optical element having a refractive index distribution along the radial direction thereof permits correction of curvature of field and aberration, including chromatic aberration, which has been impossible for nonspherical lenses. In particular, the correction of chromatic aberration is extremely advantageous for lens systems used under white light, such as a camera and a microscope. The correction of chromatic aberration largely depends on the characteristics (dispersion distribution) of the medium of the GRIN lens, and hence not all GRIN lenses have desirable ability to correct chromatic aberration.
The dispersion distribution is classified into two major categories on the basis of the direction of line corresponding to the characteristics of the gradient index optical element, as drawn on the n.sub.d (refractive index)-.nu..sub.d (Abbe's number) diagram of the appended drawing. In this connection, reference is made to U.S. Pat. No. 5,171,344. One of the two major categories is "high distributed dispersion" or "dispersion of B DIRECTION (indicated on the n.sub.d -.nu..sub.d diagram)" as exhibited by SELFOC (registered trademark) lens. The other is "dispersion of A DIRECTION (indicated on the n.sub.d -.nu..sub.d diagram)" which includes "negative distributed dispersion" along the direction perpendicular to the B DIRECTION and "low distributed dispersion" along nearly vertical directions close to the B DIRECTION.
In the dispersion distribution of the B DIRECTION, an intense chromatic aberration occurs in the same direction as that brought about by surface refraction. On the other hand, in the dispersion distribution of the A DIRECTION, a chromatic aberration occurs either significantly weakly in the same direction or in the reverse direction. In order to make an optical design fully utilizing the characteristics of a gradient index optical element, it is advantageous to employ an optical element having the characteristics of the A DIRECTION, i. e. , conducting chromatic aberration behavior which is different from that of surface-occurring chromatic aberration.
The glass composition for realizing a gradient index optical element having the above characteristics and a large refractive index gradient .DELTA.n is disclosed in U.S. Pat. No. 5,166,827. The process for producing the gradient index optical element having the above characteristics is disclosed in U.S. Pat. No. 5,171,344. In the gradient index optical element disclosed therein, two types of metals are incorporated in predetermined proportions, and have concentrations distributed in mutually reverse directions.
The glass composition disclosed in U.S. Pat. No. 5,166,827 is excellent from the viewpoint that desired optical characteristics are attained. However, the composition does not always ensure stable production, and, especially, stable, low-cost production on a large commercial scale is difficult. For example, when La or Y being a rare earth element is used, vitrification in a silicate glass system is difficult, so that small articles for tests would be producible but production of large-aperture articles would be extremely difficult. Further, it is likely for the glass to have a thermal expansion coefficient distribution along the radial direction, by which the glass may have different shrinkage ratios at the time of cooling subsequent to firing in the finishing step of the sol-gel process, so that tensile stress occurs in the surface portion of the glass to cause breakage thereof.